1. Field of the Invention
The present invention relates to an imaging device.
2. Description of the Related Art
It is commonly considered that the S/N ratio and the signal dynamic range are important product indices in solid-state imaging devices. With regard to these indices, Japanese Patent Laid-Open No. 2004-015701 provides an amplification circuit, which controls the gain applied to the pixel signal for each pixel and a detection circuit, which detects the level of a pixel signal to each pixel column of a pixel array formed by arraying pixels in a two-dimensional matrix, thereby improving the above-mentioned indices.
Also, Japanese Patent Laid-Open No. 6-070222 provides an amplification circuit which amplifies a signal output from an image sensing element. So-called “inlaid” composition is performed for a pixel signal from a relatively light region using data obtained by A/D-converting a signal output from the image sensing element, and for a pixel signal from a relatively dark region using data obtained by A/D-converting a signal amplified at a higher gain. Supposedly, this technique effectively uses the dynamic range of the imaging device.
However, in the technique disclosed in Japanese Patent Laid-Open No. 2004-015701, since a detection circuit for detecting a pixel signal from a pixel is provided for each pixel column, the area occupied by the solid-state imaging device increases, and the power consumed when the detection circuit performs detection also increases. Assume that a variable gain amplifier which uses a feedback capacitor and a switch is employed, and a MOS transistor is employed as the switch. In this case, in changing the gain for each pixel, as a charge moves upon the ON/OFF operation of the MOS transistor, the offset voltage accompanying a signal output from the amplification circuit which amplifies a pixel signal fluctuates. When this happens, upon switching the gain of the amplifier, the signal charge stored in the feedback capacitor does not completely move to another feedback capacitor used, thus generating a residual charge. This poses a problem that the gain design value becomes different from the actual gain. However, the technique disclosed in Japanese Patent Laid-Open No. 6-070222 can neither improve the S/N ratio of the signal output from the image sensing element nor widen the dynamic range of the image sensing element.